The invention relates to a cathode-ray tube comprising in an evacuated envelope an electron gun to generate an electron beam which is focused on a target, which electron gun comprises, centered along an axis, a cathode, a grid having an aperture, and a first anode having an aperture, after which grid an astigmatic cross-over is induced in the electron beam.
Such cathode-ray tubes are used to display television pictures or are used in an oscilloscope. In such cases the target is a display screen having a phosphor layer, for example in a black-and-white display tube or in an oscilloscope tube, or having a pattern of phosphor elements luminescing in different colours in a colour display tube.
Such a tube may also be used for recording pictures. In that case the target is a photosensitive layer, for example a photoconductive layer.
In all applications the spot formed when the electron beam impinges on the target must have predetermined, generally small, dimensions and the haze surrounding the spot should be minimized.
A cathode-ray tube such as that described in the opening paragraph is known from the article "30AX Self-aligning 110.degree. in-line color TV display" in IEEE Transactions on Consumers Electronics, Vol. CE-24 No. 3, August 1978, pp. 481-487. This article describes a triple electron gun in a colour television display tube in which the grid of each gun consists of two plates arranged against each other, one plate having a horizontal slot and one plate having a vertical slot. Through these slots the grid, in cooporation with the cathode, forms a first electrostatic quadrupole lens field, and in cooperation with the first anode, forms a second electrostatic quadrupole lens field rotated 90.degree. with respect to the first electrostatic quadrupole field. The electron beam is focused in two focal lines by said lens fields so that the mutual repelling of the electrons (space charge repelling) becomes less than in the case of one concentrated stigmatic cross-over. The strength of the fields between the grid and the cathode and between the grid and the anode and as a result of this the shape of the spot on the display screen, however, depend on the voltage variations at the grid. Moreover, in a grid of such a construction electron emission occurs from a non-circular region of the emissive layer of the cathode, which is undesirable in a number of applications.
U.S. Pat. No. 3,217,200 disclosed a cathode-ray tube in which a thin permanent magnetic plate is mounted against the first anode of a similar electron gun. This plate maintains a strong magnetic field through the aperture in the plate of the first anode, which magnetic field is torroidal and forms a circular symmetrical magnetic lens for the electron beam. With increasing and decreasing beam current the cross-over moves away from the cathode and towards the cathode, respectively. Thus the cross-over in this magnetic lens will move along the beam axis and the focusing influence of said lens on the beam and cross-over will vary in accordance with the beam current. Because the cross-over is displayed on the display screen, changing beam currents will result in varying spot dimensions on the display screen.